This invention relates to a method of forming a fluorescent screen of a color picture tube in which phosphor powders in a dry state are coated on the face plate of the color picture tube in a so-called dry process.
For the purpose of causing respective picture elements of a picture to emit fluorescent lights of three primary colors, the fluorescent screen of a color picture tube is so constructed as to comprise phosphor films of three colors in which dots or stripes of the phosphors are coated in a regular pattern. One method of coating in disclosed in U.S. patent application Ser. No. 895,372 filed by Saburo Nongaki et al on Apr. 11, 1978 and assigned to the same assignee.
According to the method disclosed therein, a film of photosensitive substance is coated on the inner surface of a face plate, and the portions of the film at which phosphors of a predetermined color are to be coated are exposed to light through a shadow mask to utilize a photoreaction created therein for the purpose of coating the phosphors. More particularly, a film of a photosensitive composition containing a diazonium salt as a major constituent is coated on the inner surface of the face plate and portions of the film at which the phosphors are to be coated are exposed to light. Then, zinc chloride formed in the exposed portions by photoreaction absorbs moisture in the atmosphere to become sticky so that when a powder of phosphors for emitting desired fluorescent colors is blasted onto the inner surface of the face plate, the phosphor powder will adhere only to the portions of the photosensitive film that have become sticky. After performing blasting and sticky coating of respective phosphors of three colors, the surface of the photosensitive film is treated with ammonia gas to render the portions of the photosensitive film to which the phosphor powder has been adhered insoluble in water, thereby fixing these portions. Then, surface remainders are washed away with an organic solvent thus forming a fluorescent screen. This method is one of the so-called dry types, and when compared with a conventional method in which phorsphors of respective colors are formed as photosensitive slurries which are sequentially coated, exposed, washed with water and developed, this dry process is advantageous in that only one step of coating a film of the photosensitive composition is sufficient, that it is possible to blast the phosphors in the form of a powder, and that surplus phosphor powders that had not adhered can be blown away with air spray for recovery purpose, thus increasing the efficiency of utilization of the phosphors. However, in the prior art method, there needs an independent step of rendering sticky portions at which the phosphors are to be coated insoluble in water for the purpose of fixing these portions.